Clippy lint · Restriction · checked with Clippy 0.1.99

clippy::print_stderr: use of `eprintln!`

What it meansChecks for printing on stderr. The purpose of this lint is to catch debugging remnants. It is in the restriction group and off by default.

What you see

warning: use of `eprintln!`
 --> main.rs:3:1
  |
3 | eprintln!("Hello world!");
  | ^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#print_stderr
  = note: requested on the command line with `-W clippy::print-stderr`

Real output of Clippy 0.1.99 (Rust 1.99) on the example below. Names and line numbers in yours will differ.

Does it matter?

People often print on stderr while debugging an application and might forget to remove those prints afterward.

Lints that forbid language features for specific codebases (no unwrap, no indexing...). Off by default; pick them one by one, never the whole group.

How to fix it

Clippy does not fix this one automatically. Change the code by hand:

Flagged

eprintln!("Hello world!");

Turn it off, or on

It is off by default. To use it, set it to warn or deny:

Whole crate (Cargo.toml)

[lints.clippy]
print_stderr = "warn"

In code

#![warn(clippy::print_stderr)]

One run

cargo clippy -- -W clippy::print-stderr

The [lints] table needs Rust 1.74 or newer. In a workspace, put it under [workspace.lints.clippy] and add [lints] workspace = true to each member.

Settings in clippy.toml

allow-print-in-tests: Whether print macros (ex. println!) should be allowed in test functions or #[cfg(test)]

Restrictiongroup

allowdefault level

nofixed by cargo clippy --fix

1.50.0added in Rust

Clippy's own documentation

What it does

Checks for printing on stderr. The purpose of this lint is to catch debugging remnants.

Why restrict this?

People often print on stderr while debugging an application and might forget to remove those prints afterward.

Known problems

Only catches eprint! and eprintln! calls.

Example

eprintln!("Hello world!");

Configuration

allow-print-in-tests: Whether print macros (ex. println!) should be allowed in test functions or #[cfg(test)]

(default: false)

From the Clippy lint list (0.1.99), part of the Rust project, MIT or Apache-2.0. Official entry.

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